GB2207325A - Telephone with intercom facility - Google Patents

Abstract

A telephone T1, T2???TN is provided with a relay R1, R2???RN - controlled switch SI for connecting the telephone automatically to an intercom line LI when the telephone is in the idling mode, thus providing a low cost intercom facility which avoids the need for a centralised switching network. To provide secure communication between telephones, each telephone is provided with an intercom command key which is disabled when key of a first telephone, say T1, in the system is depressed. Access codes are then transmitted by keys of the telephone T1 to activate the speech network of other telephones T2???TN as required. <IMAGE>

Description

TELEPHONE WITH INTERCOM FACILITY The present invention relates to a telephone, and more particularly to a telephone for a telephone system having an intercom facility for communication between telephones of the system without using the toll or public telephone line.

It is known to have an intercom facility between the several telephones connected to a key system private branch exchange. To communicate between the telephones it is normally necessary only to dial a particular number. A drawback of this system is that the intercom facility is routed through the private exchange, requiring the associated connecting cable and switching in the exchange. Other, older, small scale systems allow communication between a central or operators telephone, which normally receives incoming calls, and one or more subsidiary telephones to which calls are eventually routed. The need òr communication between the operators telephone and subsidiary telephone being signalled by a-call button.

A first aspect of the present invention provides a telephone having means for connecting automatically the telephone to an intercom line when the telephone is in the idle mode. The telephone is connected when it can receive or send signals from or to the intercom line to allow a call to another telephone connected to the line.

The telephone of the invention is particularly advantageous for domestic use. It is common for a dwelling to have a 4 wire telephone cable on the premises to allow the use of two telephone numbers in the dwelling. However, commonly only one number is used although socket outlets may be provided at several locations, allowing several telephones to use the one number. With the telephone of the present invention, the second, unused wire pair may be utilised as the intercom line.

Where a spare wire pair is not available, a wire pair may be run to connect up the telephones in the system without the need to run the connections to a central exchange.

Another aspect of the invention provides a control system for a plurality of telephones connected to an intercom line, said system comprising, in each telephone, enabling means for enabling the respective telephone to access other telephones in the system, and detecting means for detecting when another telephone has been enabled, said detecting means disabling the respective telephone when it detects that another telephone has been enabled to prevent the respective telephone accessing other telephones in the system.

Another aspect of the invention provides a control system for a plurality of telephones connected to an intercom line, said system comprising, in each telephone, means for sending a disabling code to other telephones in the system, means for receiving and detecting a disabling code from another telephone in the system and means for disabling the respective telephone when a disabling code is detected.

Yet another aspect of the invention provides a telephone having means for detecting the voltage level on a telephone line and means for disabiling the telephone when the voltage level drops below a predetermined value.

Other preferred features and advantages of the invention will be apparent from the following description and the accompanying claims.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates schematically a system comprising a plurality of telephones according to the invention; Figure 2 illustrates another embodiment of the invention; and Figure 3 shows a system for signalling between telephones over an intercom line.

Figure l shows schematically a telephone system having three input telephone lines L1, L2, L3 each using a wire pair and n telephones T1, T2, .......

TN. The speech and dialling circuit of each telephone T is connectable to a selected line Ll, L2 or L3 by means of a switch S as is generally known in the art to receive an incoming call or to make an outgoing call.

In accordance with the invention, each telephone Tl, T2,....TN is provided with a relay controlled switch SI for connecting the speech and dialling circuit to an intercom line LI.

The telephones Tl, T2....TN each have a central processing unit CPU as is common in the art. The CPU is configured to detect when the handset of the telephone is hung up (and when the telephone is not being used in the hands free mode if that is available), i.e. to detect when the telephone is in the idling mode. On detecting the idling mode, the respective relay Rl, R2. . RN is activated to switch the respective switch SI to connect the speech and dialling circuit to the invention line -LI.

The CPU is also configured so that if the handset is lifted, or the hands free button activated and the switch S activated to select a line Ll, L2 or L3, i.e. to receive or make an outside call, the relay R is deactivated to connect the speech and dialling circuit to the selected line Ll, L2, L3.

It can be seen then that each telephone Tl, T2. . TN operates in the usual manner but has the added feature of a relay controlled switch SI to switch the telephone to the intercom line LI.

The telephone ringing circuit remains connected to the telephone line to alert a user that there is an incoming call. A separate ringing or audible tone circuit is provided for operation to indicate an incoming intercom call.

A system for communicating between the telephones Tl, T2....TN along the intercom line LI will be described hereinafter with reference to the embodiment of Figure 3.

Figure 2 shows a system suitable for domestic premises, having a wire pair LI for incoming and outgoing calls and an intercom line LI. As mentioned previously, domestic premises or dwellings are often wired with a cable and sockets having 2 wire pairs, the second pair allowing for the connection of a second telephone line, and this second pair may be used for the intercom line when a second telephone line is not provided.

A pair of transistors Ql, Q2 is used to connect automatically each telephone Tl, T2, . . TN to the intercom line LI, when the telephone is in the idling mode. A respective transistor Ol is switched on when the associated telephone is connected to the line Ll, which switches off transistor Q2.When the telephone is in the idling mode Ql is switched off, transistor Q2 being turned on to connect the telephone to the intercom line LI. The telephone Ll is connected to each telephone via a rectifier bridge circuit B to protect the line and allowing one wire 12 of the intercom line LI to be common to both the intercom line and the telephone line feed direct to the telephone, thus requiring only one switch Ql/Q2 per telephone.

Figure 3 illustrates a control system for controlling communication to the telephones connected to the intercom line. Figure 3 shows two telephones Tl, T2 connected to an intercom line LI with a power supply PS. Each telephone Tl, T2 is provided with an intercom key Kl, and a plurality of station access keys Si, S2...SN each dedicated to a respective one of the other telephones in the system.

The telephones Tl, T2 are each controlled by a CPU housed in the telephone. The telephones are shown switched to the intercom line LI.

A level detecting means 13 is provided between the positive side of the power supply PS and an input to the CPU. The level detecting means comprises a zener diode in series with the base of a transistor.

A current consuming means 14 comprising a transistor gate is provided to draw current from the intercom line on receiving a high signal from the CPU, and the normal telephone speech network is represented by numeral 15. The gate 14 serves to turn on the speech network and draw current from the line LI.

A light emitting diode is associated with each means 13, 14.

Key K1 is enabled only when the level of signal z from the level detecting means 13 is high. When level Z goes low the key K1 is disabled, and also the associated light emitting diode is illuminated to show that K1 is disabled.

With all telephones in the idling mode level X is low and level Z is high.

To call another telephone in the system from, say, telephone T1, the respective intercom key K1 is pressed, the telephone then entering an intercom mode. After key K1 is pressed a station access key is depressed to access the desired station, for example key S2 is depressed to access telephone T2.

When key K1 for say telephone T1 is pressed the respective level X goes high, causing the current consumable means 14 to draw current from the line, thus sending the voltage level on the line LIa low, the level detecting means 13 for each telephone detects the fall in voltage level and sends a low level signal Z to the CPU, thus disabling the keys K1 for telephones T2...TN. The desired access station key(s) is (are) then depressed on telephone T1.

Depressing the key(s) instructs the CPU to send an access signal code along the line LI. When the CPU of the respective telephone T2....TN detects its access code, the CPU sends line X high, which enables the speech circuit 15 of the respective telephone.

The CPU is also arranged to generate a tone to signal that there is an incoming call in the intercom mode.

Accordingly line X goes high when either (a) key K1 is depressed (with signal Z high) or the correct access code has been received by the CPU. When line X goes high the associated light emitting diode is illuminated to show that the telephone is enabled for speech in the intercom mode.

When one intercom key K1 is depressed all other intercom keys K1 are disabled, and so a telephone speech circuit is enabled only when its respective CPU receives the correct access code.- Several station access keys can be pressed on the call originating telephone T1 to set up a conference call between three or more parties.

The system may also include means in each telephone for sending a disabling code to other telephones in the system. A disabling code may be sent to all other telephones in the system automatically when a key K1 is pressed, the disabling code being detected by the CPU of the other telephones and so disabiling them. This system can therefore replace the level detecting means 13. The speech circuit of the telephones to be accessed is then enabled by sending a respective access code as described previously.

The voltage level detecting means 13 for signalling the CPU to disable the telephone may also be used in telephones connected in parallel to a telephone line. When a plurality of telephones is connected to a common telephone line, if one telephone is off hook there will be a detectable voltage drop on the telephone line. This may be detected by a level detecting means 13 in the other telephones, the level detecting means 13 disabling the telephone to prevent other parties listening in on the line. This provides a simple, cheap way of securing the line against wlocalw eavesdropping.

Various modifications may be made and it is desired to include all such modifications as falI within the scope of the invention as defined in the accompanying claims.

Claims (23)

1. A telephone having means for connecting automatically the telephone to an intercom line when the telephone is in the idling mode.

2. A telephone as claimed in claim 1, wherein said automatic connecting means comprises a relay controlled switch.

3. A telephone as claimed in claim 1 or 2, having a central processing unit having means for detecting when the telephone is in the idling mode and means for controlling the automatic connecting means.

4. A telephone as claimed in claim 1, wherein the automatic connecting means comprises a solid state switch in series between the telephone and the intercom line, the switch being turned on when the telephone is in the idling mode and turned off when the telephone is off hook.

5. A telephone as claimed in claim 4, wherein the telephone includes a bridge rectifier circuit.

6. A telephone system comprising a plurality of telephones and an intercom line, each telephone having means for connecting the telephone automatically to the intercom line when the telephone is in the idling mode.

7. A system as claimed in claim 6, wherein the intercom line is a two wire pair.

8. A system as claimed in claim 6 or 7, wherein the automatic connecting means comprises a relay controlled switch.

9. A system as claimed in claim 8, wherein each relay is controlled by a microprocessor housed in the respective telephone.

10. A system as claimed in claim 9, wherein the microprocessor has means for detecting when the respective telephone is in the idling mode and means for controlling the automatic detecting means.

11. A system as claimed in claim 6 or 7, wherein each telephone is connected to the or a wire of the intercom line by a solid state switch, the switch being turned on when the telephone is on hook or entered into the intercom mode.

12. A system as claimed in claim 11, wherein each telephone is connected to a two wire telephone line via a rectifier bridge circuit.

13. A control system for a plurality of telephones connected to an intercom line, said system comprising, in each telephone, enabling means for enabling the respective telephone to access other telephones in the system, and detecting means for detecting when another telephone has been enabled, said detecting means disabling the respective telephone when it detects that another telephone has been enabled.

14. A control system as claimed in claim 13, further comprising, in each telephone, means for sending an access code to other telephones in the system.

15. A control system as claimed in claim 14, wherein said means for sending an access code is enabled when the respective telephone is enabled by the enabling means.

16. A control system as claimed in claim 13, 14 or 15, wherein said detecting means comprises means for detecting a voltage level on said intercom line, and said enabling means causes said voltage level to change from a first level to a second level when a telephone is enabled, said detecting means detecting when the voltage level is at said second level.

17. A control system as claimed in claim 14, 15 or 16 wherein each telephone has an access code detection means for detecting an access code specific to the respective telephone, and second enabling means for enabling the speech circuit of the telephone to connect the telephone to the intercom line when the specific access code is detected.

18. A control system for a plurality of telephones connected to an intercom line, said system comprising, in each telephone, means for sending a disabling code to other telephones in the system, means for receiving and detecting a disabling code from another telephone in the system and means for disabling the respective telephone when a disabling code is detected.

19. A control system as claimed in claim 18, wherein the disabling code is common to all telephones in the system.

20. A telephone substantially as hereinbefore described with reference to Figure 1, Figure 2 and/or Figure 3 of the accompanying drawings.

21. A telephone system substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

22. A control system for a plurality of telephones connected to an intercom line, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

23. A telephone having means for detecting the voltage level on a telephone line and means for disabling the telephone when the voltage level drops below a predetermined value.